1. Field of the Invention
The present invention relates to a display device and a method for fabricating the display device, and more particularly relates to a method for repairing a pixel defect.
2. Description of the Prior Art
Liquid crystal display devices are characterized by having a thin body and low power consumption. Utilizing such characteristics, liquid crystal display devices are widely used for personal computer, cellular phones and the like. Specifically, an active matrix type liquid crystal display device including a switching element such as a thin film transistor (which will be hereafter referred to as a “TFT”) provided for each pixel (i.e., the smallest image unit) can reliably turn ON each pixel, and thus allows for fine moving picture display.
The liquid crystal display device includes an active matrix substrate in which a plurality of pixel electrodes are arranged in a matrix, a counter substrate provided so as to be opposed to the active matrix substrate and including a common electrode, and a liquid crystal layer interposed between the substrates.
FIG. 14 is a plan view schematically illustrating an active matrix substrate 60 constituting a general liquid crystal display device.
In the active matrix substrate 60, a plurality of gate lines 1 and a plurality of source lines 2 are provided so that the gate lines 1 intersect with the source lines 2 at right angles. TFTs 5 are provided so that each of the TFTs 5 is located at each of intersections of the gate lines 1 and the source lines 2. Capacitor lines 3 are provided so that each of the capacitor lines 3 is located between adjacent ones of the gate lines 1 so as to be parallel to the gate lines 1. Furthermore, pixel electrodes 8 are provided so that each of the pixel electrodes 8 corresponds to one of the TFTs 5 and is located in a display region surrounded with a pair of gate lines 1 and a pair of source lines 2.
A TFT 5 includes a gate electrode G, i.e., a protruding portion of a gate line 1 and a source electrode S, i.e., a protruding portion of a source line 2, and a drain electrode D provided so as to face the source electrode S.
The drain electrode D is extended so as to form a connection line 16 and an auxiliary capacitor electrode 15 and connected to a pixel electrode 8 through a contact hole 15a. Moreover, the auxiliary capacitor electrode 15 and a capacitor line 3 overlap with each other with an insulation film interposed therebetween to form an auxiliary capacitor.
In the liquid crystal display device, when an image is displayed, a gate signal is sent from a predetermined gate line 1 to turn ON a TFT 5 connected to the gate line 1 and at the same time, a source signal is sent from a source line 2 to write predetermined electric charges in a pixel electrode 8 through the source electrode S and the drain electrode D, so that a potential difference is generated between the pixel electrode 8 and a common electrode of the counter substrate and a predetermined voltage is applied to a pixel capacitor formed of a liquid crystal capacitor of a liquid crystal layer and the auxiliary capacitor. A transmittance of incident light from the outside is adjusted by changing an alignment state of liquid crystal molecules forming the liquid crystal layer with the applied voltage to display an image.
In the liquid display device having the above-described structure, a voltage applied during a period in which a TFT is in an ON state has to be stored in a pixel capacitor for a certain amount of time. In many cases, if the pixel capacitor is formed of only a liquid crystal capacitor, the applied voltage is reduced due to a liquid crystal and a leakage current of the TFT to cause an insufficient storage operation or the pixel capacitor is influenced by a parasitic capacitance. Therefore, to suppress reduction in an applied voltage in the pixel capacitor, in addition to the liquid capacitor, an auxiliary capacitor is generally provided so as to be electrically parallel to the liquid crystal capacitor.
A capacity of the auxiliary capacitor can be increased by increasing an area occupied by the capacitor line 3 and the auxiliary capacitor electrode 15 in each display region, reducing the thickness of the insulation film between the capacitor line 3 and the auxiliary capacitor electrode 15 or the like. With the increase in the capacity of the auxiliary capacitor, the storage property of the pixel capacitor can be improved, so that display quality of the liquid crystal display device is increased.
However, if in order to increase the capacity of the auxiliary capacitor, the areas of the capacitor line 3 and the auxiliary capacitor electrode 15 are increased and the thickness of the insulation film therebetween is reduced, the pixel capacitor becomes easily influenced by particles (particulate contaminants), dusts and the like, which can be possibly attached to a substrate surface in process steps for fabricating a liquid crystal display device, so that a short circuit might occur between the capacitor line 3 and the auxiliary capacitor electrode 15. In such a case, a potential of the capacitor line 3 is directly applied to the pixel electrode 8, so that a failure of pixel display, i.e., a defective pixel might be caused at increased possibility.
In a liquid crystal display device having the above-described problems, for example, dotted or linear defects are detected in display inspection. Normally, in a liquid crystal display device, a predetermined number of dotted defects or less are acceptable. However, if the number of defects is increased, the liquid display device is judged to be a defect.
Therefore, conventionally, techniques for repairing such dotted defective pixels have been proposed and put into use in fabrication of a liquid crystal display device.
For example, Japanese Laid-Open Publication No. 2-108028 describes an example in which an auxiliary capacitor is formed between a capacitor line and a pixel electrode. In Japanese Laid-Open Publication No. 2-108028, disclosed is an active matrix substrate in which a plurality of protruding potions are provided in a capacitor line in advance so that the protruding potions intersect with the capacitor line with right angles and which is so configured that when a short circuit occurs between any one of the plurality of protruding portions and a pixel electrode, laser is irradiated to a base portion (connection portion) of the protruding portion corresponding to part in which the short circuit has occurred to cut off the connection portion for defect repair.
Moreover, Japanese Laid-Open Publication No. 2001-330850 describes an example in which an auxiliary capacitor is formed between a capacitor line and an auxiliary capacitor electrode. In Japanese Laid-Open Publication No. 2001-330850, disclosed is a liquid crystal display device in which an auxiliary capacitor electrode is divided into a plurality of regions in advance and the regions are connected with one another by connection portions each having a shape to allow defect repair in a simple manner and which is so configured that when a short circuit occurs between any one of the plurality of regions of the auxiliary capacitor electrode and a capacitor line, laser is irradiated to a connection portion corresponding to part in which the short circuit has occurred to cut off the connection portion for defect repair.
In each of Japanese Laid-Open Publication No. 2-108028 and In Japanese Laid-Open Publication No. 2001-330850, when a short circuit occurs in the auxiliary capacitor electrode, laser is irradiated to a metal thin film forming the capacitor line and a metal thin film made of the same material as that for a drain electrode and forming the auxiliary capacitor electrode to cut off the connection portion.
However, to form interconnects and electrodes with excellent conductivity, the thickness of the metal thin film forming the connection portion has to be large. With a large thickness, the metal film can not be cut off in a simple manner and thus the pixel electrode, a common electrode of a counter substrate and the like might be damaged by the cut off. Furthermore, in a liquid crystal display device with interconnects and TFTs whose sizes have been reduced more and more, an interconnect pattern and the like in the periphery of the metal thin film might be damaged by cutting off the metal thin film of the connection portion.